Plasma arc torches and methods of operating and testing the same

ABSTRACT

A plasma arc torch comprises relatively displaceable electrode and nozzle components having first and second positions relative to one another in which the electrode respectively contacts the nozzle and is spaced an operating distance therefrom. Arrangements are disclosed for relatively displacing the electrode and nozzle between the first and second positions thereof with a pre-determined force of engagement therebetween in the first position, and without having to interrupt the flow of arc gas and/or the flow of arc current during such relative displacement. A torch start up safety test procedure is performed by sequentially displacing the electrode and nozzle between the two positions thereof, sensing for appropriate open and closed circuit conditions therebetween, and enabling the creation of a pilot arc only when a pre-determined sequence of circuit conditions is sensed. Sequential displacement of the electrode and nozzle together with the application of a pre-determined force of engagement therebetween promotes cleaning of oxides from the area of contact between the electrode and nozzle.

BACKGROUND OF THE INVENTION

The present invention relates to the art of plasma arc torches and, moreparticularly, to improvements in connection with the starting andre-starting of such torches as well as the preliminary testing andcleaning and the efficient and safe operation thereof.

It is of course well known that a plasma arc torch comprises anelectrode and a nozzle which are relatively displaceable between aposition in which the electrode contacts the nozzle and a position inwhich the electrode is spaced an operating distance from the nozzle. Thenozzle provides a pilot arc chamber having a plasma outlet opening and,when the electrode contacts the nozzle, an arc current is flowed betweenthe electrode and nozzle, a plasma or arc gas is supplied to the pilotarc chamber, and a pilot arc is created as the electrode moves away fromthe nozzle to its operating position. As the electrode moves away fromthe nozzle, it uncovers the plasma outlet opening, whereby a plasma jetis emitted from the nozzle providing for the torch to operate in anon-transferred pilot arc mode. By moving the nozzle into proximity withthe workpiece, the arc is transferred thereto and the torch thenoperates in the arc-transferred mode which is preferred for cuttingmetal.

Many arrangements have been provided heretofore for relativelydisplacing the electrode and nozzle for creating the pilot arc, and anumber of such arrangements are shown, for example, in U.S. Pat. Nos.3,004,189 to Giannini; 3,242,305 to Kane et al; 4,791,268 to Sanders etal; 4,896,016 to Broberg et al; 4,902,871 to Sanders et al; 5,164,569 toPorra et al; and 5,208,441 to Broberg, which patents are incorporatedherein by reference for purposes of background information. In the twopatents to Sanders et al and the patent to Porra et al, the initialposition of the electrode is the position in which the electrodecontacts the nozzle and displacement of the electrode from contact withthe nozzle to create the pilot arc requires use of the arc gas flowingtoward the pilot arc chamber as the primary source of power fordisplacing the electrode relative to the nozzle. More particularly, inthese arrangements the electrode is structurally associated with apiston against which the pressurized arc gas is applied to force theelectrode away from the nozzle against the biasing force of a spring bywhich the electrode is moved into engagement with the nozzle when thearc gas is shut off to de-pressurize the piston chamber. In the patentto Broberg, the initial position of the electrode is that in which theelectrode contacts the nozzle, and displacement of the electrode awayfrom the nozzle is achieved through a piston and cylinder arrangementseparate from and connected to the electrode through a mechanicallinkage arrangement. In Broberg, the supply of arc gas to the torch istapped to provide the motive fluid for displacing the piston against thebias of a spring which operates through the linkage arrangement toreturn the electrode into contact with the nozzle when the pistonchamber is de-pressurized. In one embodiment shown in Kane et al, theinitial position of the electrode is the operative position thereof inwhich the electrode is spaced from the nozzle, and the electrode isassociated with a piston by which the electrode is displaced intocontact with the nozzle by applying the arc gas against one side of thepiston. Such displacement is against the return bias of a spring bywhich the electrode returns to its initial position when the pistonchamber is de-pressurized. The pilot arc is created during the lattermovement of the electrode, and cooling water under pressure is imposedon the opposite side of the piston to hold the electrode in theoperative position during operation of the torch.

Arrangements of the foregoing character have a number of disadvantages.In this respect, for example, in the patents to Sanders et al and thepatents to Porra et al and Broberg at least the arc gas supply if notboth the latter and the arc current circuit must be interrupted in orderto begin a new pilot cycle. Thus, if the arc is unintentionallyextinguished during operation of the torch, the torch cannot bere-started without interrupting arc gas flow. This undesirably increasesthe cycle time for creating the pilot arc in that, following arcextinction, the operator must wait for the gas supply to shut off andfor the power source for the torch to be reactivated and for the gassupply to come back to operating pressure before the pilot arc can bestarted again. A further disadvantage resides in the fact that, for arealistically sized torch, the available active surface area for thepiston is limited such that the spring that forces the electrode againstthe nozzle must be of a low force. Thus, if the electrode does not touchthe nozzle with sufficient force, a pilot arc may not be created becauseof a build-up of oxides on the electrode and/or nozzle. The limitedpressure of the arc gas also limits the latter spring force. Moreover,with the arangmenets in the two patents to Sanders et al and the patentto Broberg, it is impossible to have a pre-flow fo arc gas such as forcleaning the nozzle-electrode area, or a post-flow of arc gas, such asfor cooling the nozzle-electrode area.

In the embodiment of Kane et al referred to, restarting of the torchrequires interrupting the power supply in order to evacuate the coolingwater from the piston chamber so that the electrode can be displacedinto contact with the nozzle. Accordingly, the same undesirably longcycle time attaches to this arrangement in Kane et al. In Broberg et al,a lever and over-center spring mechanism provides for manualdisplacement of the electrode from its initial position in contact withthe nozzle during generation of the pilot arc, and the over-centerspring holds the electrode in its operative position. Release of theover-center spring mechanism snaps the electrode back to its positionengaging the nozzle and, while the impact of the electrode with thenozzle provides a cleaning action, release of the over-center springmechanism requires interruption of the torch power supply. Suchinterruption is also required to intentionally extinguish the arc at theend of a working operation. Therefore, restarting of the torch followingunintentional extinction of the arc requires interruption of the powersupply and thus an increase in the cycle time between successive starts.Furthermore, it is impossible to have a pre-flow of arc gas prior toinitiating generation of the arc, or to have a post-flow of arc gasfollowing an intentional extinction of the arc. Still further, the leverand over-center spring mechanism of Broberg et al, like the lever andpiston arrangement of Broberg, require a considerable number ofstructural components in addition to the electrode, and a disposition ofmany of the components laterally of the electrode and nozzle axes. Suchstructural complexity results in an undesirable amount of frictionbetween the moving parts and the potential for the application of sidethrust in connection with displacement of the electrode, all of whichcan affect the ease and reliability of electrode movement.

In Giannini, the electrode in its initial position contacts the nozzleand is spring biased thereagainst, and a solenoid connected to the powersource for the torch is operable to displace the electrode away from thenozzle during generation of the arc. Intentional extinction of the arcrequires interrupting the power supply whereupon the electrode returnsto its initial position engaging the nozzle. Restarting of the torchfollowing an unintentional extinction of the arc also requiresinterrupting the current source so that the electrode returns to itsinitial position. Thus, the cycle time for restarting is undesirablyhigh, and there can be no pre-flow or post-flow of arc gas prior togenerating the pilot arc or following intentional extinction of the arc.

In another embodiment shown in Kane et al, the electrode is springbiased into contact with the nozzle and cooling water for the nozzle andelectrode acts against a piston connected to the electrode to displacethe electrode from contact with the nozzle and against the spring biasduring generation of the pilot arc. Return of the electrode to itsinitial position by the biasing spring requires interruption of thepower supply in order to reduce the cooling water pressure. Accordingly,restarting of the torch following an unintentional extinction of the arcrequires interruption of the power supply and this, together with thetime required for reducing the water pressure, results in an undesirablyhigh cycle time between successive starts of the torch. Furthermore, itis not possible to have a pre-flow of arc gas prior to generating thepilot arc or a post-flow following intentional extinction of the arc inthat the latter requires interruption of the power supply and thusreturn of the electrode into contact with the nozzle.

Other problems attendant to the starting and operation of plasma arctorches heretofore available relate to the breaking up and removal ofoxide build-up on the electrode and/or nozzle to assure good electricalcontact between the latter in connection with generating a pilot arc,and the ability to efficiently, effectively and economically perform apreliminary safety check to assure that the nozzle is in place on thetorch and is in proper assembled relationship with the torch body andthus the electrode. Such a safety check is desirable to prevent theoperator of the torch from getting accidentally shocked such as bycontacting a tool with the electrode if the latter is exposed such aswould be the situation if the nozzle is not in place on the torch. Atthe same time, no or improper contact of the electrode with the nozzlemay occur if the nozzle is on the torch but improperly oriented relativeto the electrode. The latter can result in the inability to generate astarting arc and/or damage to the nozzle and/or electrode during arcgeneration as a result of the improper electrical contact therebetween.Heretofore, arrangements for sensing the presence of the nozzle on atorch has required various contacts, springs and the like separate fromand in addition to the torch operating components.

SUMMARY OF THE INVENTION

In accordance with the present invention, the foregoing and otherdisadvantages and problems encountered in connection with plasma arctorches heretofore available are advantageously minimized or overcome.More particularly in this respect, and in accordance with one aspect ofthe invention, the electrode and nozzle components of a torch arerelatively displaceable between initial and second positions so as toprovide, selectively, for pre-flow and/or post-flow of arc gas and so asto enable restarting of a torch following unintentional extinction ofthe arc without interrupting the power supply or the flow of arc gas tothe pilot arc chamber, thus to minimize the cycle time betweensuccessive starts. The initial position of the electrode may be eitherthe position in which the electrode contacts the nozzle or the positionin which the electrode is spaced an operating distance therefrom and, ineither event, the electrode can be displaced from the latter position tocontact the nozzle without interrupting the flow of arc gas to the pilotarc chamber, thus to facilitate restarting the torch in the event thearc is unintentionally extinguished during a cutting operation. Moreparticularly in this respect, an operating force for relativelydisplacing the nozzle and electrode from its operative position intocontact with the nozzle is operable independent of the flow of arc gasto the pilot arc chamber and independent of the arc current powersupply. The operating force may, for example, be provided by anelectro-mechanical device such as a solenoid, an operating gas sourceacting against a piston and which source can be separate from the arcgas source or tapped therefrom. Further, the operating force can beprovided in the latter manner in combination with a spring, and can beprovided as well by other devices capable of imparting relativedisplacement between the electrode and nozzle such as thermallyexpandable and contractible memory wire, thermally responsive bi-metaldisks and the like. Advantageously, the component or components by whichthe operating force is applied are arranged co-axially with respect tothe electrode and nozzle axes, thus optimizing reliability of electrodemovement relative to the nozzle and the ability to overcome dirt andminimize friction between moving parts of the torch.

Further in accordance with this aspect of the invention, the operatingforce can be of a pre-determined magnitude which advantageously pushesor impacts the electrode against the nozzle so as to break up any oxidebuild up on either one or both of the electrode and nozzle components.Moreover, the engaging force promotes good electrical contact and thusoptimizes generation of a pilot arc upon separation of the electrodefrom the nozzle with arc current applied therebetween. Advantageously,the magnitude of the engaging force between the electrode and nozzle canbe several times greater than that possible with prior art arrangementswherein the engaging force is limited substantially to that of a biasingspring or a force generated by the arc gas, both of which are limited asa result of the required working pressure of the arc gas and therelatively small size of the electrode. Still further in accordance withthis aspect of the invention, the electrode and nozzle can be relativelydisplaced into and out of contact with one another a number of timesprior to generating the starting arc, thus to optimize the cleaningoperation and, advantageously, such multiple displacement can beachieved with a flow of arc gas to the pilot arc chamber so as to carryparticles of the broken up oxide out of the pilot arc chamber throughthe plasma outlet opening in the nozzle. Such displacement capabilitywith the flow of arc gas to the pilot arc chamber and without generatinga starting arc upon relative displacement of the electrode and nozzleout of engagement with one another is enabled by providing for therelative displacement between the electrode and nozzle to be independentof the arc current power supply and by providing for the force componentby which the electrode engages the nozzle to either overcome the forceby which the electrode is displaced away from the nozzle or by removingthe latter force. As mentioned hereinabove, the force applyingarrangements are co-axial with the electrode and nozzle whereby theforegoing capabilities and advantages are achieved while minimizing theusage of space within the torch body.

In accordance with another aspect of the invention, a system is providedfor performing a preliminary safety check or test by which generation ofa pilot arc is enabled only if the nozzle is mounted on the torch andproperly oriented relative to the electrode, thus to protect an operatorfrom being accidentally shocked and to preclude operation of the torchuntil it is safe to do so. Further in accordance with this aspect of theinvention, the safety check is performed using the electrode and nozzleas primary components of the test, thus eliminating the need formodifying the structure of the torch to accommodate testing. Minimaltesting is achieved in accordance with the invention by positioning theelectrode to contact the nozzle, applying a test voltage between thenozzle and electrode and sensing for a closed circuit condition whichwill exist only if the nozzle is on the torch and properly positionedfor contact with the electrode and the electrode and/or nozzle are freeof any oxide build up which would preclude a closed circuit condition.If a closed circuit condition is sensed, the system enables the flow ofarc current between the electrode and nozzle, whereby a pilot arc isgenerated upon relative displacement of the electrode and nozzle awayfrom one another. If a closed circuit condition is not sensed, thesystem prevents the flow of arc current between the electrode andnozzle. Once the condition precluding the sensing of a closed circuit iscorrected, contact of the nozzle and electrode will result in thesensing of a closed circuit and operation of the torch will be enabled.

A safety check in accordance with this aspect of the invention can bemade when the initial position of the electrode is spaced from thenozzle or in contact therewith. If the electrode is initially spacedfrom the nozzle, the electrode and nozzle can be relatively displacedfor the electrode to engage the nozzle at which time the test voltage isapplied therebetween in the foregoing manner. Preferably, however, thetest procedure includes sensing for an open circuit condition when theelectrode is initially spaced from the nozzle and enabling thegeneration of a pilot arc only if an open circuit is sensed followed bythe sensing of a closed circuit when the electrode contacts the nozzle.Furthermore, when the initial position of the electrode is the positionin which the electrode contacts the nozzle it is preferred tosequentially relatively displace the electrode and nozzle from theinitial position to the spaced position and then back to the initialposition, sequentially sensing for closed, open and closed circuitconditions, and enabling the generation of a pilot arc only if theappropriate sensed circuit conditions correspond to the sequence ofelectrode and nozzle positions. Assuming the torch nozzle to be removed,such a closed-open-closed test requirement would be extremely difficultif not impossible for an operator to duplicate such as by shorting theelectrode to nozzle connection with a tool. Similarly, with the initialposition of the electrode being the position in which the latter isspaced from the nozzle, it is preferred to sequentially relativelydisplace the electrode and nozzle from the initial position at leasttwice to the position of contact with the nozzle, thus to perform anopen-closed-open-closed test which would be impossible to duplicate.Further in connection with any of such testing procedures, furtherprotection against an operator accidentally duplicating the test ispreferably achieved by timing the sensed circuit condition orconditions. In any event, it will be appreciated that the electrode andnozzle can be relatively displaced into and out of engagement any numberof times with a pre-determined sequence of open and closed circuitconditions being required to be sensed in order to enable generation ofa pilot arc.

In accordance with yet another aspect of the invention, testing in theforegoing manner can be achieved independent of or in conjunction withcleaning of the electrode-nozzle area of contact as described above. Inthis respect, for example, the electrode and nozzle can be pulsed forthe electrode to contact the nozzle one or more times to remove anybuild up of oxide on the nozzle and/or electrode, after which the testprocedure would be initiated. Such a preliminary cleaning would optimizeobtaining a closed circuit condition upon contact between the nozzle andelectrode during the test procedure. At the same time, however, it willbe appreciated that in conjunction with performing a safety test, thepulsing of the electrode and nozzle between the positions of engagementand disengagement will cause any build up of oxide to be broken up andtherefore, that the required current condition signal or signals toenable starting of the torch will eventually be sensed.

It is accordingly an outstanding object of the present invention toprovide improvements in connection with the starting, restarting of aplasma arc torch and the efficient and safe operation thereof.

Another object is the provision of a plasma arc torch wherein electrodeand nozzle components are relatively displaceable between a position ofcontact and a position in which the electrode is spaced an operatingdistance from the nozzle without having to interrupt the arc currentsupply or the flow of arc gas to the plasma arc chamber of the torch.

Yet another object is the provision of a plasma arc torch whereinrestarting of the torch following arc extinction can be achieved withouthaving to interrupt the arc current or the flow of arc gas to the pilotarc chamber.

Still another object is the provision of a plasma arc torch wherein theelectrode and nozzle can be relatively displaced into engagement with apredetermined engaging force therebetween and which force has amagnitude greater than that which can be applied when the arc gassupplied to the pilot arc chamber in combination with a biasing springcontrols relative displacement of the electrode and nozzle between theengaged and disengaged positions thereof.

Still another object is the provision of a plasma arc torch capable ofselectively enabling pre-flow and post-flow of arc gas to the pilot arcchamber independent of the arc current power supply.

A further object is the provision of a plasma arc torch wherein relativedisplacement of the electrode and nozzle from an operating position ofthe electrode to a position in which the electrode contacts the nozzleis achieved by overcoming or eliminating the force by which theelectrode and nozzle are maintained in the operating position of theelectrode and without having to interrupt the flow of arc gas or theflow arc current to overcome or eliminate the maintaining force.

A further object is the provision of a plasma arc torch with improvedability for breaking up the build up of oxides on the electrode and/ornozzle components.

A further object is the provision of a plasma arc torch in which theelectrode and nozzle components are adapted to be relatively displacedinto and out of engagement independent of the generating of a pilot arcto provide for cleaning the area of contact between the electrode andnozzle prior to initiating operation of the torch.

Yet a further object is the provision of an improved method and systemfor testing a plasma arc torch for the presence of a nozzle thereonand/or appropriate orientation of the nozzle relative to the torchelectrode.

Another object is the provision of a method and system of the foregoingcharacter which employs the electrode and nozzle as primary componentsin the test system.

Still another object is the provision of a method and system of theforegoing character which is operable to control the flow of arc currentto the electrode and nozzle and which is operable to enable generationof a pilot arc only in response to the occurrence of a predeterminedtest parameter.

Still another object is the provision of a plasma arc torch in which theelectrode and nozzle are relatively displaceable between positions ofcontact and separation and are operable in a manner which avoids thetime heretofore required for arc gas and/or arc current power recoverybetween sequential starts of the torch.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects, and others, will in part be obvious and in partpointed out more fully hereinafter in conjunction with the writtendescription of preferred embodiments of the invention illustrated in theaccompanying drawings in which:

FIG. 1 is a sectional elevation view of a plasma arc torch in accordancewith the present invention;

FIG. 2 is a block diagram schematically illustrating a system inaccordance with the present invention for testing a plasma arc torch forthe presence of a nozzle;

FIG. 3 is a flow chart of the operation of the testing system;

FIGS. 3A-3C are signal, air flow and arc pattern graphs corresponding todifferent formats of operation of the test system;

FIGS. 4-7 are pattern graphs illustrating the occurrence of variousfunctions which take place during different test formats;

FIG. 8 is a sectional elevation view somewhat schematically illustratinganother arrangement for relatively displacing the electrode and nozzlecomponents of a plasma arc torch in accordance with the presentinvention;

FIG. 9 is a sectional elevation view schematically illustrating stillanother arrangement for relatively displacing the electrode and nozzlecomponents of a plasma arc torch in accordance with the invention;

FIG. 10 is a sectional elevation view illustrating still anotherarrangement for relatively displacing the electrode and nozzlecomponents of a plasma arc torch in accordance with the presentinvention;

FIG. 11 is a sectional elevation view of a further arrangement forrelatively displacing the electrode and nozzle components of a plasmaarc torch in accordance with the invention;

FIG. 12 is a sectional elevation view schematically illustrating yet afurther arrangement for relatively displacing the electrode and nozzlecomponents of a plasma arc torch in accordance with the invention;

FIG. 13 is a pattern graph illustrating force and arc gas flowcharacteristics applicable to the arrangement shown in FIG. 12; and

FIG. 14 is a sectional elevation view illustrating a plasma arc torchelectrode and nozzle arrangement according to the prior art.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now in greater detail to the drawings wherein the showings arefor the purpose of illustrating preferred embodiments of the inventiononly and not for the purpose of limiting the same, FIG. 1 illustrates aplasma arc torch 10 having a generally cylindrical body portion 12supporting an electrode 14 and a nozzle 16 having a plasma outletopening 18. Nozzle 16 is supported on body portion 12 by a swirl ring 20of insulating material and an internally threaded cap member 22 providedon its outer side with an insulating shield 24. Body portion 12 supportsa cylindrical member 26 of insulating material which, together with anupper cap member 28 of insulating material provides a piston chamber 30which is co-axial with the electrode and nozzle. A piston member 32 isaxially slidably supported by cylindrical member 26 and end cap 28 andincludes an upper end 34 disposed in chamber 30. The upper end ofelectrode 14 is secured to the lower end of piston member 32 fordisplacement with the piston member, and the upper end of the pistonmember includes stem portion 36 slidably extending through an openingtherefor in end cap 28. Annular seal members 38, 40 and 42 are providedabout stem 36, upper end 34 and the portion of piston member 32 belowchamber 30, respectively, for the purpose which will become apparenthereinafter. Stem 36 is provided with an axial exhaust passageway 44having laterally extending inner ends 46 opening into chamber 30 beneathupper end 34 of piston member 32, and end cap 28 is provided with anaxially extending airflow passageway 48 having an inner end opening intochamber 30 above upper end 34. Passageway 48 is connected to a source ofoperating air under pressure, not shown, for displacing piston member 32and thus electrode 14 downwardly relative to nozzle 16 in the manner andfor the purpose set forth hereinafter. Body portion 12 is provided withan arc air inlet opening 50 connected to a source of arc air underpressure, not shown. The source of operating air can be separate fromthe source of arc air or can be tapped therefrom. Body portion 12 andcylindrical member 26 provide an annular arc gas passageway 52, andannular member 26 is provided with radially extending ports 54connecting passageway 52 with an inner annular passageway 56 surroundingelectrode 14 and communicating with pilot arc chamber 58 between thelower ends of electrode 14 and nozzle 16. Electrode 14 and nozzle 16 areadapted to be connected across a source of arc current by leads 60 and62, respectively, whereby, in a well known manner, when electrode 14 isdisplaced downwardly from the position shown in FIG. 1 to contact nozzle16 and is then moved back toward the position shown in FIG. 1 a pilotarc is generated therebetween.

Displacement of electrode 14 downwardly from the position shown in FIG.1 to contact nozzle 16 is achieved by flowing operating air underpressure into passageway 48 and thus against the upper side 35 of pistonmember 32 and exhausting the air beneath upper end 34 through passageway44. Displacement of the electrode from contact with nozzle 16 back tothe position shown in FIG. 1 is achieved by flowing arc gas underpressure through passageway 54 and thus against the underside 64 ofpiston member 32 while allowing for the air introduced throughpassageway 48 to exhaust through the latter passageway. As will becomeapparent hereinafter, electrode 14 is displaceable downwardly from theposition shown in FIG. 1 to contact nozzle 16 without having tointerrupt the flow of arc air through inlet opening 50 and is sodisplaceable against the force generated in the opposite direction bythe arc air acting against the piston member. In this respect, as willbe appreciated from FIG. 1, upper side 35 of piston member 32 provides areaction surface of considerably greater area than that of surface 64 onthe underside of the piston member. Thus, assuming for example that thearc air and operating air are at the same given pressure, the forceacting in the downward direction in FIG. 1 against piston member 32 isgreater than the force acting in the upper direction whereby, inaccordance with one aspect of the invention, electrode 14 can bedisplaced downwardly from the position shown in FIG. 1 to contact nozzle16 without having to interrupt the supply of arc air through inletopening 50. This advantageously enables restarting of the torch in theevent of an unintentional extinguishing of the arc without having tointerrupt the arc current power supply and/or arc air flow as heretoforerequired. Moreover, such displacement capability advantageously lends toa pre-flow of arc air to the nozzle and through the outlet opening 18 soas to blow any dirt or the like out of the pilot arc chamber, andpost-flow of the arc gas following intentional extinction of the arcsuch as for cooling purposes. At the same time, it will be appreciatedthat electrode 14 can be displaced in opposite directions relative tonozzle 16 by alternately flowing operating air through one of thepassageways 44 and 48 and exhausting operating air through the other andwithout any flow of arc gas through inlet opening 50, whereby pre-flowand post-flow of arc gas is selective. Further in connection withcleaning, it will be appreciated that electrode 14 can be displaceddownwardly to contact nozzle 16 with a pre-determined force sufficientto break up any oxide build-up on the electrode and/or nozzle and can bedisplaced into and out of contact with the nozzle a number of times forthis purpose. The force of contact between the electrode and nozzle isof course dependent on the area of the upper surface of piston member 32and the pressure of operating air acting thereagainst, whereby it willbe appreciated that a desired force of engagement between the electrodeand nozzle can readily be achieved by varying these parameters.

Further in connection with FIG. 1, it will be appreciated that theelectrode and nozzle in a plasma arc torch have an initial positionrelative to one another which can be the position in which the electrodecontacts the nozzle or the position in which the electrode is spaced anoperating distance therefrom. The embodiment of FIG. 1 advantageouslyenables either of the relative positions between the electrode andnozzle to be the initial position. Important too is the fact that all ofthe components parts by which the foregoing advantages are achieved areco-axial with respect to the electrode and nozzle axes, thus minimizingthe space requirements for the component parts.

In accordance with another aspect of the present invention, as will beappreciated from the following description of FIG. 2-7 with reference tothe electrode and nozzle arrangement of FIG. 1, a safety check system isprovided for assuring that nozzle 16 is properly mounted on the torchbody relative to electrode 14 before enabling starting of the torch.More particularly in this respect, as shown in FIG. 2, lines 60 and 62respectively connect electrode 14 and nozzle 16 to power source 66 toenable creating a pilot arc, and a workpiece W is connected to the powersource by a line 68 to facilitate transfer of the arc to the workpiece.Line 60 is provided with a safety switch 70 for opening and closing thecircuit to electrode 14 and nozzle 16 and for activating a safetycircuit 72 such as in response to depression of the torch trigger. Acontroller 74 includes a micro-processor programmed to respond to sensedtest parameters and to control the test procedure as well as enabling ordisabling creation of a pilot arc as will be described in greater detailhereinafter. A force generating unit 76 by which electrode 14 isdisplaced relative to nozzle 16 is adapted to be activated by controller74 and, in conjunction with the electrode and nozzle arrangement of FIG.1, force generating unit 76 would be defined by a source of operatingair under pressure.

A test procedure in accordance with this aspect of the invention isdepicted in the flow chart of FIG. 3 and the pattern graph of FIG. 3A.In this procedure, the operator depresses the torch trigger to start thesafety check by closing switch 70 to activate safety circuit 72 whichthen sends a signal through line 78 to activate controller 74. While notshown in FIG. 2, depression of the torch trigger can also initiate flowof arc gas to the arc gas chamber 58, if desired. Controller 74 sensesthat the test voltage to be applied from power source 66 to theelectrode and nozzle is on and, assuming the initial relative positionbetween electrode 14 and nozzle 16 to be the spaced apart position shownin FIG. 2, controller 74 sends a control signal through line 80 to powersource 66 and in response to which power source 66 outputs a low voltagetest signal through the circuit defined by line 60, electrode 14, nozzle16 and line 62. The power source then senses the electrode-nozzlecircuit condition and sends a corresponding signal through line 82 tocontroller 74. An open circuit condition produces a high voltage signal,and a closed circuit condition produces a low voltage signal. In thespaced position of electrode 14 relative to nozzle 16, an open circuitcondition should be sensed. If an open circuit condition is not sensedsuch as due to a short between the electrode and nozzle, controlleroutputs a control signal through line 84 to safety circuit 72 by whichthe latter opens switch 70, thus precluding further operation of thetorch until the fault condition is corrected. If an open circuitcondition is sensed, controller 74 outputs a control signal through line86 to force generating unit 76 by which the latter responds to displaceelectrode 14 downwardly from the position shown in FIG. 2 into contactwith nozzle 16. Power source 66 then senses for a closed circuitcondition and transmits a signal corresponding to the sensed circuitcondition to controller 74 through line 82. If a closed circuitcondition is not sensed, such as when there is a build-up of oxides onthe electrode and/or nozzle, controller 74 outputs a control signalthrough line 84 to safety circuit 72 by which switch 70 opens topreclude further operation of the torch until the fault condition isrectified. If a closed circuit condition is sensed by power source 66,controller 74 outputs a control signal through line 80 to power source66 enabling an arc starting current to flow in the electrode-nozzlecircuit, and the controller outputs a signal through line 88 to forcegenerating unit 76 by which the latter operates to displace electrode 14to its position spaced an operating distance therefrom, whereby an arcis generated as the electrode moves out of contact with the nozzle asindicated by arrow 90 in FIG. 3A.

While the safety start sequence described above enables the power sourcefor creating a starting arc upon the sequential sensing of an opencircuit condition followed by a closed circuit condition, it will beappreciated as shown in FIG. 3, that the sequence of open and closedcircuit condition checks can be repeated any number of times inaccordance with the program of the micro-processor in controller 74prior to enabling the power source to supply an arc voltage to theelectrode-nozzle circuit. A preferred starting sequence in this respectwith the initial position of the electrode being that in which theelectrode is spaced from the nozzle is depicted in FIG. 3B. As will beappreciated from the preceding description, the micro-processor incontroller 74 would in this instance be programmed to require thesequential sensing of open, closed, open and closed circuit conditionsbefore enabling starting of the arc following the sensing of the secondclosed circuit condition.

While the foregoing starting sequence safety checks are described inconjunction with the initial position of the electrode being that inwhich the electrode is spaced from the nozzle, a starting sequencesafety check with the electrode contacting the nozzle in the initialposition thereof is depicted in FIG. 3C. In this instance, themicro-processor of controller 74 would be programmed to control thestarting of an arc in response to the power source sequentially sensinga number of closed and open circuit conditions and, preferably, as shownin FIG. 3C, a sequence of at least closed, open and closed circuitconditions. As mentioned above, and as will be further appreciated fromFIG. 3C, the pre-flow of arc gas is optional as is the point at whichpre-flow is initiated which, in FIG. 3C, is at the point where theelectrode is first displaced from its initial position to its positionspaced from the nozzle.

Starting sequence safety checks of the foregoing characteradvantageously optimize safety for the operator in the event thatstarting of the torch is attempted when, for example, nozzle 16 is notmounted on the torch, whereby the electrode is exposed, whereby theoperator can accidentally short the electrode across a conductiveportion of the torch body such as with a tool. A sequentialelectrode-nozzle circuit condition test according to the invention wouldbe difficult to defeat in requiring a specific sequential conditionwhich would have to be matched by the operator's actions in shorting theelectrode. Further in accordance with this aspect of the invention,protection against such duplicating is optimized by providing for eachof the open and closed circuit conditions to have a pre-determined timeof duration as indicated by times T1, T2, T3 and T4 in FIGS. 3A-3C, andwhich time duration may, for example, be about 200 milliseconds or less.

FIGS. 4-7 schematically illustrate patterns of displacement of variousparts of the torch during test starting sequences having different testformats. FIGS. 4-6 depict such patterns in connection with test formatssimilar to those depicted in FIGS. 3B, 3C and 3A, respectively, and FIG.7 illustrates such patterns in connection with yet another test format.While these figures will be discussed with reference to thenozzle-electrode arrangement illustrated in FIG. 1 and the use ofoperating air under pressure as the medium for providing upward anddownward force for displacing the electrode relative to the nozzle, itwill become apparent hereinafter that other electrode-nozzlearrangements in accordance with the invention provide variouscombinations of air, springs and the like for providing the upward anddownward forces.

Referring now to FIG. 4, which as mentioned above, correspondssubstantially to the starting sequence described above with respect toFIG. 3B, and considering the retracted position of electrode 14 in FIG.1 to be the initial position in which the electrode is held by thefrictional engagement between seals 38, 40 and 42 and piston member 32,the pattern of the starting sequence is as follows. The torch trigger isinitially off and there is no airflow through either passageway 44 or 48from the operating air source, whereby the downward force against pistonhead 34 is low and the upward force thereagainst is off. Electrode 14 isretracted relative to nozzle 16, the arc air pressure is off, althoughit could be on as described here-in-above with regard to FIG. 3B. Thereis no contact between electrode 14 and nozzle 16, and the plasma currentis off as is the starting arc. At point P1, the torch trigger is turnedon to activate safety circuit 72 and thus controller 74 of the testsystem, whereupon the open circuit condition between the electrode andnozzle would be sensed as explained above. At point P2, following thesensing of the open circuit condition, controller 74 issues a controlsignal through line 86 by which force generating unit 76 is operable toinitiate flow of operating air through passageway 48 against the upperside of piston head 34, whereby the downward force goes high andelectrode 14 is displaced downwardly to contact nozzle 16 at point P3.At point P3, the closed circuit condition is sensed and controller 74 isoperable in response thereto to activate the upward force output fromforce generating unit 76 for operating air under pressure to flowthrough passageway 44 to the underside of piston head 34, whereby theupward force is turned on and, at point P4, controller 74 is alsooperable to turn on the arc air pressure, whereupon a pre-flow of thearc air enters the torch through inlet opening 50. As explained abovewith regard to FIG. 1, the operating force acting downwardly on pistonhead 34 is greater than that acting upwardly against the underside ofthe piston head, whereby at point P4 the electrode remains in contactwith the nozzle. At point P5, following the sensing of the closedcircuit condition, force generating unit 76 receives a control signalfrom controller 74 through line 88, whereupon the force generating unitis operable to interrupt the flow of operating air under pressurethrough passageway 48 so as to depressurize the upper side of pistonchamber 30, whereupon the downward force against piston head 34 goeslow, electrode 14 moves back to its retracted position at point P6 andthe electrode-nozzle are again in the open circuit condition. It will benoted at this point that the arc air pressure and the upward forceagainst the underside of piston head 34 are maintained on constantlyfrom point P4. At point P5 when electrode 14 moves out of contact withnozzle 16, an open circuit condition exists and is sensed as describedhereinabove and, at point P7 force generating unit 76 is operable inresponse to a control signal from controller 74 to flow control airunder pressure through passageway 48 against the upper side of pistonhead 34, whereby the downward force again goes high to displaceelectrode 14 downwardly to contact nozzle 16 at point P8, whereuponsensing of the closed circuit condition resulting therefrom commences.Assuming that the system has sensed the appropriate sequence of open,closed, open and closed circuit conditions at this time, the plasmacurrent is turned on at point P9 and, at point P10, force generatingunit 76 is operable in response to a control signal from controller 74to once again exhaust the upper end of chamber 30 whereupon the downwardforce goes low and the upward force operates to displace electrode 14 toits retracted position. In conjunction with the latter displacement ofelectrode 14, a pilot arc is initiated at point P10 as electrode 14moves out of contact with nozzle 16.

As mentioned above, FIG. 5 depicts a start up test sequencecorresponding substantially to that described hereinabove with regard toFIG. 3C, whereby it is believed that the function patterns depicted inFIG. 5 will be understood from the description of FIG. 3C and theforegoing description of FIG. 4 without a step-by-step description ofFIG. 5. It only needs to be noted that in FIG. 5 the initial position ofthe electrode is the position in which the electrode contacts thenozzle. Thus, with the electrode-nozzle arrangement illustrated in FIG.1, electrode 14 would initially contact nozzle 16 and would be held inthe latter position by the frictional engagement between the seals andpiston member. Further, at point P2 in FIG. 5, force generating unit 76operates in response to a control signal from controller 74 to cause theflow of air through passageway 48 against the upper side of piston head34, whereby the downward or closing force goes high to assure goodelectrical contact for the sensing of the initial closed circuitcondition which takes place between points P3 and P5.

As also mentioned above, FIG. 6 corresponds substantially to thestart-up test sequence described here-in-above with regard to FIG. 3Awherein the initial position of the electrode is the position shown inFIG. 1 in which the electrode is spaced an operating distance from thenozzle. Thus, with the exception of the patterns for the plasma currentand arc, the patterns in FIG. 6 correspond to those discussed here-in-above with regard to FIG. 4, the difference being that starting of thepilot arc in FIG. 6 is enabled upon the sequential sensing of one opencircuit condition followed by one closed circuit condition, namely atpoint P5. Accordingly, the plasma arc current in FIG. 6 is turned on atpoint P3A following detection of a closed circuit at point P3, and thearc is initiated at point P5 rather than at point P10 as in FIG. 4.

The starting sequence patterns depicted in FIG. 7 are similar to thosedepicted in FIG. 5 with the exception of the plasma current and arcpatterns. The patterns depicted in FIG. 7 correspond to a torch such asthat in FIG. 1 in which the initial position of the electrode is thatposition in which the electrode contacts the nozzle whereby, as in FIG.5, the initial electrode-nozzle circuit condition is closed. In FIG. 7,the pilot arc is enabled if a closed circuit condition is sensed withthe electrode contacting the nozzle in the initial position of theelectrode and, therefore, the plasma arc current is turned on at pointP1A to enable the arc to be generated at point P5 when the electrode isdisplaced out of engagement with the nozzle.

With further regard to the patterns depicted in FIGS. 4-7, it will beappreciated that the exact time of starting the flow of arc gas is notcritical, provided that the flow commences some time between when thetorch trigger is pulled and the electrode is retracted from contact withthe nozzle at the point in the sequence at which generation of the pilotarc is to take place.

With further regard to FIGS. 4-7, and in accordance with another aspectof the invention, it will be appreciated that cleaning of the area ofcontact between the electrode and nozzle in accordance with theinvention advantageously takes place during the described start upsafety tests. More particularly in this respect, each time the electrodecontacts the nozzle, the high downward force operates to break downoxide build up on the electrode and/or nozzle. Moreover, as describedhere-in-above and as will become more apparent hereinafter, the downwardforce applied to the electrode can be several times greater than theforce available in accordance with prior art arrangements, thusoptimizing the cleaning capability. As will be appreciated from thosesafety tests wherein the electrode is displaced into and out ofengagement with the nozzle more than once, each such displacementenhances the cleaning operation. As will be further appreciated from thedescription thus far, the ability to selectively provide for thepre-flow of arc gas during the test procedures further enhances thecleaning operation by blowing broken up oxide particles from the pilotarc chamber. At the same time, it will be appreciated that cleaning ofthe area of contact between the electrode and nozzle can be achieved byrelatively displacing the electrode and nozzle into and out of contactwith the application of a force of engagement therebetween to promotecleaning but independent of the testing procedure. In this respect, forexample, the electrode could be displaced into and out of engagementwith the nozzle a number of times before initiating a test procedure,thus to assure good electrical contact between the electrode and nozzleduring a subsequent testing operation. Still further in connection withtesting and cleaning operations, if the start sequence is disabledduring testing because a closed circuit condition is not sensed with theelectrode positioned to contact the nozzle, with the nozzle properlymounted on the torch body, a cleaning operation can be performed bydisplacing the electrode into and out of contact with the nozzle anumber of times prior to initiating a subsequent starting sequencesafety check. All of these capabilities advantageously enhanceminimizing the cycle time between sequential starts of a torch andmaintenance time otherwise required to disassemble, clean and reassemblethe nozzle and electrode components.

Another electrode and nozzle arrangement by which the foregoingadvantages and operating capabilities are achieved is illustrated inFIG. 8 of the drawing. In this embodiment, torch body 100 supports anannular member 102 of insulating material which provides a pistonchamber having an upper portion 104 and a lower portion 106 of reduceddiameter relative to portion 104. The piston chamber reciprocallysupports a piston member having a piston head 108 in chamber portion 104and a stem 110 in chamber portion 106. Piston head 108 and stem 110carry sealing rings 112 and 114, respectively, by which the pistonmember is sealed relative to the piston chamber. The upper end ofchamber portion 104 is closed by an end wall 116, and the piston memberis normally biased to the position shown in FIG. 8 by a tension spring120 between the upper end of piston head 108 and wall 116. An electrode122 is mounted on the lower end of stem 110 for reciprocatingdisplacement with the piston member, and body portion 100 of the torchremoveably supports a nozzle member 124 having a plasma outlet opening126 therethrough. Arc air is adapted to be supplied to pilot arc chamber128 from a suitable source of air under pressure, not shown, through aline 130, a passageway 132 provided in body portion 100 and radial ports134 provided in the lower end of cylindrical member 102. Operating airunder pressure is adapted to be selectively introduced into upperchamber portion 104 above piston member 108 through a passageway 136 inend wall 116 and a supply line 138 connected to arc air supply line 130and having an operating air control valve 140 interposed therein. Valve140 has a deactivated position as shown in FIG. 8 in which the portionof chamber 104 above piston head 108 is vented to atmosphere through aline 142, and an activated position in which the valve slide isdisplaced to the left in FIG. 8 so as to connect line 138 thereacrossand to block the flow to atmosphere through line 142. Valve 140 can bepneumatically displaced from its deactivated position to its activatedposition and includes an operating portion 144 for this purpose, and thevalve is biased to its deactivated position by a spring 146. Chamberportion 104 beneath piston head 108 is vented to atmosphere by means ofa port 148. The electrode and nozzle are adapted to be connected to asource of arc current as schematically illustrated by lines 150 and 152,respectively.

As will be appreciated from FIG. 8 and the above description, theinitial position of electrode 122 relative to nozzle 124 is the positionin which the electrode is spaced an operating distance from the nozzleand which position is provided by the bias of tension spring 120. Theoperating force for displacing electrode 122 downwardly to contactnozzle 124 is provided by air under pressure flowing through line 138across valve 140 and inlet passage 136 against the upper side of pistonhead 108. More particularly in this respect, when arc air is beingsupplied to pilot arc chamber 128 through supply line 130, valve 140 canbe selectively displaced to the left in FIG. 8 such that a portion ofthe arc air flows through line 138 to inlet passageway 136 and the upperend of chamber portion 104 so as to act against the piston head 108. Aswill be appreciated from FIG. 8, the reaction surface of piston head 108as defined by upper surface 108A thereof has an area considerably largerthan that of the underside 110A of stem 110 against which arc gasentering through passageway 134 may react, whereby the piston member andthus electrode 122 are rapidly displaced downwardly for the electrode toimpact against nozzle 124 and to apply a closing force thereagainstseveral times greater than the return force provided by spring 120 andreaction of the arc air against surface 110A. Thus, with a relativelylow arc gas pressure, a considerably high closing force can be appliedwith the arrangement shown in FIG. 8. Moreover, while this arrangementadvantageously provides for obtaining such a high closing force usingthe pressure of the arc air supply for achieving the desired result, itwill be appreciated that the operating fluid for displacing the pistonhead and thus electrode 122 into contact with nozzle 124 could beprovided from a source other than the arc air supply. In any event, itwill be appreciated that valve 140 can be sequentially cycled betweenthe positions thereof to sequentially displace electrode 122 intocontact with and away from contact with nozzle 124 either for cleaningand/or in connection with start sequence testing as describedhereinabove. Advantageously, the arc gas supplied to theelectrode/nozzle area through supply line 130 is not relied upon to biaselectrode 122 into either of its operating positions, and any bias ofthe electrode in the direction to move the electrode away from thenozzle is easily overridden by the closing force provided by the arc gasacting against piston surface 108A. The latter advantageously enablesrestarting the torch in the event of an unintentional extinction of thearc during operation of the torch. In this respect, following suchextinction and without having to interrupt the flow of arc current inthe electrode/nozzle circuit, valve 140 can be displaced to the left inFIG. 8 for arc air to flow through line 138 into the upper end ofchamber 104 to displace piston head 108 and thus electrode 122downwardly for the electrode to contact nozzle 124, after which valve140 is released for return to the position shown in FIG. 8, whereuponelectrode 122 moves out of contact with nozzle 124 such that the pilotarc is generated to restart the torch.

FIG. 9 illustrates yet another electrode and nozzle arrangement inaccordance with the present invention. In this embodiment, electrode 154is attached to the underside of a piston 156 reciprocally supported by acylindrical member 158 of insulating material to which a nozzle 160 isattached. The upper end wall 162 of member 158 is apertured to slidablyreceive a force applying member 164 having a lower end 166 which is notattached to piston 156 and accordingly is adapted to be displacedrelative thereto. Basically, piston 156 and electrode 154 float relativeto member 158 and nozzle 160 in that the piston and electrode are notinitially biased other than by the negligible weight of force applyingmember 164. In this embodiment, the initial position of electrode 154 isthe position in which the electrode contacts nozzle 160 as shown in FIG.9, and the electrode is adapted to be displaced from its initialposition to a position in which the electrode is spaced an operatingdistance from nozzle 160 by arc air under pressure from a source 168thereof. Further, as will become apparent hereinafter, force supplyingmember 164 is adapted to be displaced downwardly against the upper sideof piston 156 by a coil 170 extending thereabout and adapted to beelectrically energized and de-energized through control circuitryincluding a device 172 operable in conjunction with a start sequencetesting system such as that described above with regard to FIG. 2. Insuch test system device 172 would be responsive to control signals fromcontroller 74 to sequentially energize and de-energize coil 170 inconjunction with open and closed circuit testing of the electrode-nozzlepositional relationship. The control circuitry further includes atrigger switch 174, a double pole switch 176 and a single pole switch178, the operations of which are described hereinafter, and a time delayrelay device 180 in series with one pole of switch 176. The flow of arcair from source 168 to pilot arc chamber 182 is controlled by a solenoidactuated valve 184, which is normally closed as shown in FIG. 9, and thesolenoid coil of valve 184 is adapted to be energized and de-energizedthrough a solenoid or relay switch 186 which is selectively controlledas set forth hereinafter by trigger switch 174 and double pole switch176.

In operation of the embodiment shown in FIG. 9, with regard first toperforming a start up safety test, trigger switch 174 is closed which,as will be appreciated from the foregoing description of FIG. 2,activates the safety circuit and controller, whereby device 172 wouldrespond to a control signal from the controller to energize coil 170 andapply a downward force on drive member 164 during the sensing of aclosed circuit condition in the initial position of the electrode. Theclosure of trigger switch 174 also energizes the coil of switch 186 toclose the latter, whereby the coil of valve 184 is energized to displacethe valve to its open position whereupon arc air under pressure flowsfrom source 168 to pilot arc chamber 182. Following the closed circuitcondition check, device 172 is operable to de-energize coil 170whereupon arc gas under pressure in chamber 182 acts against theunderside of piston 156 to elevate the latter and thus electrode 154 tothe operating position thereof relative to nozzle 160. Following theopen circuit check in the latter position, device 172 is again operatedto energize coil 170 whereby drive member 164 displaces the piston andelectrode downwardly for the electrode to contact nozzle 160, suchdisplacement being against the bias of arc gas against the underside ofpiston 156. Moreover, it will be appreciated that the magnitude of thedownward force which can be applied depends on the coil design, and thatthe downward force can be quickly applied so as to impact electrode 154against nozzle 160 to optimize the break up of any oxide therebetween.It will be further appreciated that the electrode can be sequentiallydisplaced in the foregoing manner between the open and closed positionsthereof relative to nozzle 160 any number of times, concluding withgenerating of a pilot arc in response to the final displacement of theelectrode out of contact with the nozzle. Should the arc beunintentionally extinguished during operation of the torch, switch 178can be closed independent of interruption of the arc current or thesupply of arc gas to energize coil 170 for drive member 164 to displacethe electrode downwardly to its position contacting nozzle 160,whereupon the torch is restarted when switch 178 is opened and theelectrode is moved out of contact with the nozzle by the arc gas actingagainst the underside of piston 156.

Switch 176 and time delay device 180 are operable to achieve a cleaningoperation with respect to the electrode and nozzle independent of theoperation of the torch to achieve a starting sequence safety check.Since cleaning of the electrode-nozzle area can be achieved inconjunction with such a safety check, it will be appreciated that thecircuit including switch 176 and time delay device 180 is optional. Inorder to perform a cleaning operation with the latter circuit, switch176 is adapted to be closed independent of trigger switch 174 to closeswitch 186 and thus open valve 184 for the flow of arc air to the pilotarc chamber, and to close a circuit to time delay device 180 which is inseries with solenoid coil 170 of the force applying arrangement. In theclosed position of switch 176, time delay device 180 is operable tointermittently energize and de-energize coil 170 so as to achieve therapid displacement of electrode 154 into and out of contact with nozzle160. It will be appreciated that the time delay device can be designedto achieve a considerable number of displacements in a relatively shortperiod of time thus to in effect repeatedly bounce the electrode againstthe nozzle to promote cleaning of oxides from therebetween.

FIG. 10 illustrates a plasma arc torch in accordance with the inventionwherein relative displacement between the torch electrode and nozzle isachieved by operation of a solenoid drive unit. More particularly inthis respect, a torch body component comprising a cylindrical upperportion 188 and a necked in cylindrical lower portion 190 supports asolenoid mounting and electrode guide assembly comprising a solenoidmounting member 192 and an electrode guide member 194. The lower end ofmember 192 rests on the annular shoulder between upper and lower bodymember portions 188 and 190, and the upper end of electrode guide member194 fictionally and wedgedly interengages with the lower end of thesolenoid mounting member. The lower end of electrode guide member 194extends slightly below the lower end of portion 190 of the torch body,and a nozzle member 196 having a plasma outlet opening 198 therein isremoveably mounted on body portion 190 by a mounting sleeve 200 whichclampingly engages the upper end of the nozzle against the lower end ofthe nozzle guide member.

A solenoid unit 202 is supported in the torch body by the upper end ofsolenoid mounting member 192 and co-axial with the torch nozzle.Solenoid unit 202 includes a downwardly extending plunger 204 which isaxially displaceable downwardly relative to the solenoid unit uponenergization of the solenoid coil and, upon de-energization of the coil,is displaceable upwardly of the solenoid unit such as by an internalbiasing spring, not shown. An electrode holder 206 has its upper endattached to plunger 204 for displacement therewith and is provided atits lower end with an electrode 208 suitably mounted thereon fordisplacement therewith. Solenoid unit 202 is adapted to be connectedacross a suitable source of electrode power through a switch 210 havingopen and closed positions respectively energizing and de-energizing thesolenoid coil. Lower portion 190 of the torch body member is providedwith an inlet opening 212 adapted to be connected to a source of arc airunder pressure, not shown, and the lower end of electrode guide 194 isprovided with radial ports 214 opening into an axially extendingperipheral passageway 216 by which arc air flowing through inlet 112flows to pilot arc chamber 218.

In this embodiment, the initial position of electrode 208 is theposition in which the electrode is spaced an operating distance fromnozzle 196 and, as will be appreciated from the foregoing description,electrode 208 is biased to the latter position by the spring in solenoidunit 202. It will likewise be appreciated that switch 210 is moved fromthe open position to the closed position thereof shown in FIG. 10 toenergize the solenoid coil and displace plunger 204 and thus electrode208 downwardly for the latter to contact nozzle 196. Moreover, it willbe appreciated that the downward force of engagement of electrode 208with nozzle 196 is a predetermined force which can be of considerablemagnitude depending on the design parameters of solenoid unit 202 as canbe the speed with which the electrode is displaced into contact withnozzle 196 to obtain an impacting force thereagainst, both of whichimpacting and force of engagement promote the breakup of oxide in thearea of engagement between the electrode and nozzle. In connection withgenerating a pilot arc between the electrode and nozzle, the lattercomponents are adapted to be connected to an arc current source by leads220 and 222, and it will be appreciated from the foregoing descriptionthat electrode 208 is displaceable between the two positions thereofrelative to nozzle 196 without having to interrupt either the arccurrent power supply or the flow of arc gas to the torch. Accordingly,as will be understood from the foregoing description, should the arc beaccidentally extinguished during operation of the torch, switch 210 canbe closed to energize solenoid unit 202 to displace electrode 208downwardly to contact nozzle 196. Thereafter, switch 210 is opened and apilot arc is generated and the torch restarted as electrode 208 is movedupwardly from contact with nozzle 196 to its operative position relativethereto. Still further, as will be appreciated from the description ofearlier embodiments herein, the torch depicted in FIG. 10 can beoperated in conjunction with a start up testing system such as thatillustrated in FIG. 2, and the area of contact between the electrode andnozzle can be cleaned either in conjunction with or through operation ofthe component parts independent of a start up safety check.

Referring now to FIG. 11, there is illustrated a torch structurallysimilar in many respects to the torch shown in FIG. 10, wherebycorresponding parts are designated in FIG. 11 by the same numeralsappearing in FIG. 10. In this embodiment, the initial position ofelectrode 208 is the position in which the electrode contacts nozzle 196as shown in FIG. 11. The upper end of electrode holder 206 is modifiedto provide the latter end with a piston member 224, and a compressionspring 226 is interposed between solenoid mounting member 192 and piston224 to exert a downward force on the electrode holder by which electrode208 is biased to contact nozzle 196 with a force of predeterminedmagnitude corresponding to the force of spring 226. Further inaccordance with this embodiment, electrode holder 206 and thus electrode208 are adapted to be displaced upwardly from the position shown in FIG.11 to a position in which the electrode is spaced in operating distancefrom nozzle 196 by air under pressure acting against the underside ofpiston member 224 in a chamber 228 therebeneath. Operating air for sodisplacing piston member 224 is adapted to be introduced into an annularchamber 230 provided in the upper end of solenoid mounting member 192,and upper portion 188 of the torch body is provided with an inletopening 232 for connecting chamber 230 with a source of operating airunder pressure, not shown. For the purpose which will become apparenthereinafter, solenoid mounting member 192 is provided with radial ports234 communicating chamber 230 with a valve chamber 236 and with radialports 238 communicating valve chamber 236 with axially extendingpassageways 240 leading to radial ports 242 opening into piston chamber228. The flow of operating air entering chamber 230 from inlet 232 iscontrolled by a valve member 244 mounted on solenoid stem 204 fordisplacement therewith and having an axially extending circumferentialrecess 246 in the outer surface thereof.

Valve member 244 is adapted to be displaced by stem 204 to the positionshown in FIG. 11 when switch 210 is closed to energize the solenoidcoil, and in this position the upper end of valve member 244 closes theradially inner ends of ports 234. When switch 210 is opened tode-energize the solenoid coil, a compression spring 248 biases valvemember 244 upwardly from the position shown in FIG. 11 to a position inwhich recess 246 provides communication between radial ports 234 and 238and in which position operating air under pressure can flow throughinlet passageway 232 and to chamber 228 beneath piston member 224,whereupon the latter is displaced upwardly to move electrode 208 out ofengagement with nozzle 196. Thus, it will be appreciated that solenoidunit 202 is de-energized during operation of the torch and during whichoperation the operating air under pressure maintains piston 224 and thuselectrode 208 in the operative position of the electrode. Because of therelatively large surface area on the underside of piston 224, the forceof spring 226 can be considerably greater than heretofore possible inarrangements where a spring is relied upon to provide the force ofengagement between the electrode and nozzle. Thus, the cleaning of theelectrode-nozzle area and good electrical contact are achieved.

The source of operating air connected to inlet 232 can be separate fromthe arc air supplied through inlet opening 212 for flow to the pilot arcchamber or can be a take off from the arc air supply. In any event, itwill be appreciated that electrode 208 can be sequentially displacedbetween its initial and operative positions relative to nozzle 196 bysequentially displacing member 244 between its two positions forallowing and blocking the flow of operating air to piston chamber 228.Furthermore, it will be appreciated that such sequential displacement ofthe electrode between its positions can be achieved without interruptingthe flow of arc gas to the torch and without interrupting the flow ofarc current whereby, in the event of an unintentional extinction of thearc during use of the torch, switch 210 can be closed to energize thecoil of solenoid unit 202 to displace valve member 244 to the positionshown in FIG. 11 blocking the flow of air to piston chamber 228whereupon spring 226 biases electrode 208 downwardly to contact nozzle196. The subsequent opening of switch 210 to de-energize the coil ofunit 202 results in displacement of valve member 244 upwardly from theposition shown in FIG. 11 to communicate the supply of operating airwith piston chamber 228, thus to displace electrode 208 from contactwith nozzle 196 enabling a pilot arc and thus restarting of the torch totake place during such upward displacement. As will be furtherappreciated from the foregoing description, sequential displacement ofelectrode 208 between its initial and operative positions relative tonozzle 196 is operable to provide cleaning of the area of contactbetween the nozzle and electrode and provides for the electrode andnozzle arrangement to be operated in conjunction with a start up testingsystem as described herein to assure against starting of the torch untilit is safe to do so.

Another electrode-nozzle arrangement in accordance with the presentinvention is illustrated in FIG. 12 wherein the electrode 250 isattached to the underside of a piston member 252 which is verticallydisplaceable in a cylindrical support component 254 of the torch body. Anozzle 256 having a plasma outlet opening 258 is mounted on the lowerend of component 254, and the latter is provided with radial inletopenings 260 for the supply of arc air to pilot arc chamber 262 from asuitable source of arc air under pressure, not shown. The initialposition of electrode 250 is the position in which the electrodecontacts nozzle 256 as shown in FIG. 12, and the electrode is biased tothe initial position by the force F_(s) of a spring 264 acting againstthe upper side of piston member 252. Electrode 250 is adapted to bedisplaced upwardly from its initial position to a position in which theelectrode is spaced an operating distance from nozzle 256 by an upwardforce F_(u) which is equal to the force F_(A2) created by arc air underpressure acting against reaction area A2 on the underside of pistonmember 252. Such upward displacement is against the force of spring 264and operating air under pressure acting against the reaction surface A1on the upper side of piston member 252 is adapted to provide a forceF_(A1) which supplements the force of spring 264 to provide apre-determined downward force F_(D) against the piston member for thereasons set forth hereinafter. In this embodiment, the reaction area A1for the upper surface of piston member 252 for such operating air underpressure is defined by a bellows component 266 which, advantageously,seals against leakage of operating air while providing for the downwarddisplacing force to be independent of any force required to overcomefrictional sealing engagement between the piston member and cylindricalcomponent 254. The arrangement in this embodiment advantageouslyprovides for a pre-determined downward force F_(D) to be applied againstpiston 252 which enables the latter to be displaced downwardly from itsoperative position to its position contacting nozzle 256 without havingto interrupt the flow of arc gas to chamber 262. Preferably, suchdownward force F_(D) as defined by the force F_(s) of spring 264 and theforce F_(A2) created by operating air pressure acting against area A1 ofthe upper surface of piston 252 within bellows 266 is at least 1.5 timesgreater than the upward force F_(u) which is the force F_(A2) created bythe pressure of arc air acting against reaction surface A2 on theunderside of piston 252.

It will be appreciated from FIG. 12 and the above description that forceF_(s) plus force F_(A1) provides pre-determined downward force F_(D) forengaging electrode 250 against nozzle 256 against the upward or openingforce F_(u) provided by force F_(A2). In order for arc air underpressure to displace piston 252 upwardly from the position shown in FIG.12 to the operative position of the electrode in connection withgenerating a pilot arc, force F_(A2) must be greater than the springforce F_(s). Similarly, for the downward force F_(D) to displace theelectrode back to its position contacting nozzle 256 withoutinterrupting the flow of arc air to chamber 262, the combination offorces F_(A2) and F_(s) must be greater than the upward force F_(A2). Asmentioned above, it is preferred that F_(A2) plus F_(s) is at least 1.5times greater than F_(A2). This relationship is achieved with theexample which accompanies FIG. 12 and in which the spring force F_(s)has a value of 1, F_(A1) has a value of 5 and F_(A2) has a value of 3.In this example, the desired relationship is 6 to 3 whereby the downwardforce F_(D) is twice the upper force F_(u).

FIG. 13 is a force pattern graph depicting the manner in which forceF_(A1) is employed in conjunction with starting and restarting thetorch, and further depicting the selectivity with respect to the time orpoint at which the flow of arc air to chamber 262 is initiated. With thecomponent parts in the position shown in FIG. 12, electrode 250 isbiased downwardly by spring force F_(s) and, prior to starting operationof the torch such as by depressing the torch trigger, operating airunder pressure is not introduced into bellows 266. Accordingly,referring to pattern line F_(A1), prior to starting of the torch forceF_(A1) is zero. Starting of the torch is initiated at point P1 and, atthis point, operating air under pressure is introduced to bellows 266whereby force F_(a1) is imposed on piston 252 together with spring forceF_(s) to apply a pre-determined downward force on electrode 250 asindicated by arrow 268. This downward force moves electrode 250 intocontact with nozzle 256 and the force of engagement provides a cleaningfunction as described herein as well as promoting good electricalcontact between the electrode and nozzle.

As shown by pattern lines F_(A2) in Example 1 and Example 2 in FIG. 13,the flow of arc air into chamber 262 can either precede or follow theinitiating of the starting procedure at point P1. As will be appreciatedfrom the description herein of the various forces, force F_(A2) is lessthan the downward force F_(D), whereby in Example 1 the downward forcewill override the upward force to displace electrode 250 into contactwith nozzle 256. In Example 2, the flow of arc air to chamber 262 afterthe application of the downward force F_(D) will have no effect withrespect to displacing electrode 250 upwardly.

At point P2, operating air under pressure in bellows 266 is exhaustedtherefrom, whereby force F_(A1) drops back to zero and force F_(A2)created by arc air under pressure in chamber 262 is operable to displacepiston 252 and thus electrode 250 upwardly from contact with nozzle 256as indicated by arrow 270. Assuming the electrode and nozzle to beconnected across an arc current source, a pilot arc is created when theelectrode moves away from contact with the nozzle. Should the arcthereafter be unintentionally extinguished during operation of thetorch, it will be appreciated from the foregoing description thatoperating air under pressure can be introduced into bellows 266 withoutinterrupting the flow of arc air to chamber 262 or interrupting the arccurrent supply. Thus, at point P3 for example, operating air underpressure is introduced into bellows 266 whereupon force F_(A1) increasesto its pre-determined value and electrode 250 is displaced downwardly tocontact nozzle 256. At point P4 the operating air in bellows in 266 isexhausted whereupon force F_(A1) returns to zero, the arc gas in chamber262 displaces electrode 250 upwardly from contact with nozzle 256 and apilot arc is generated between the electrode and nozzle to restart thetorch. As with the previous embodiments described herein, it will beappreciated that electrode 250 can be sequentially displaced between itsinitial position and its operative position relative to nozzle 256 anynumber of times in conjunction with a cleaning operation and/or a startup safety check.

It is believed that the improved operating characteristics and theadvantages achieved through the relative displacement between theelectrode and nozzle of a plasma arc torch in accordance with thepresent invention will be more fully understood from the followingdescription of a prior art arrangement illustrated in FIG. 14 of thedrawings. With reference to the latter figure, the plasma arc torchcomprises a generally cylindrical body member 272, an electrode 274, anda nozzle 276 having a plasma outlet opening 278 therein. Electrode 276includes a piston member 280 axially slidably received in a cylinderchamber 282 in body member 272, and the electrode is biased downwardlyto its initial position contacting nozzle 276 by a compression spring284. Arc gas under pressure is adapted to flow through a passageway 286and swirl ports 288 to pilot arc chamber 290, and the inlet end ofpassageway 286 is connected to a source 292 of arc gas under pressure bya flow line 294 through which flow is controlled by a solenoid valve296. The control circuitry for the torch includes a normally opentrigger switch 298 which controls a normally open relay switch 300.Switch 300 includes a contact 302 for opening and closing an electricalcircuit to the coil of solenoid valve 296, and a contact 304 for openingand closing a circuit for supplying arc current between electrode 274and nozzle 276.

In operation, trigger switch 298 is closed whereby relay switch 300 isactuated to close the valve coil and arc current circuits, whereupon arcgas valve 296 opens for arc gas to flow to pilot arc chamber 290 and arccurrent is supplied between the electrode and nozzle. Arc gas underpressure in chamber 290 acts against the underside of piston 280 todisplace electrode 274 upwardly from contact with nozzle 276 against thebias of spring 284. A pilot arc is generated as electrode 274 movesupwardly from contact with nozzle 276, and the arc gas under pressure inchamber 290 maintains the electrode in its upper position duringoperation of the torch. Should the arc be unintentionally extinguishedduring operation of the torch, both the supply of arc gas and the supplyof arc current must be interrupted in order to restart the torch. Inthis respect, it is impossible to move electrode 274 back to its initialposition contacting nozzle 276 without interrupting the flow of arc gasto pilot arc chamber 290, and such interruption of the arc gas flowrequires opening of trigger switch 298 and thus opening of the arccurrent circuit. Accordingly, the cycle time for restarting the torch isundesirably increased in that the operator must wait for the air supplyto shut off, for the power source to recover and for the air supply tocome back on before the pilot arc can be restarted. Furthermore, becauseof the relatively small area on the underside of piston 280 availablefor generating the opening force, and the fact that such force islimited by the arc gas pressure, biasing spring 284 has to be of arelatively low force to assure appropriate displacement of the electrodeto its operative position spaced from nozzle 276 and retention of theelectrode in the operative position. Accordingly, when the upward forceis removed, spring 284 does not bias electrode 274 into contact withnozzle 276 with sufficient force to assure the breaking of any oxidebuild up on the electrode and/or nozzle and to assure good electricalcontact therebetween. Thus, the nozzle must be frequently removed andthe electrode and nozzle cleaned of oxide build up in order to assuresufficient electrical contact between the electrode and nozzle toachieve generation of a starting arc.

While considerable emphasis has been placed herein on the embodimentsillustrated and described, it will be appreciated that other embodimentscan be devised and that the embodiments herein can be modified withoutdeparting from the principals of the invention. Accordingly, it is to bedistinctly understood that the foregoing descriptive matter is to beinterpreted merely as illustrative of the invention and not as alimitation.

We claim:
 1. In a plasma torch comprising relatively displaceableelectrode and nozzle means having first and second positions relative toone another in which said electrode respectively contacts said nozzlemeans and is spaced an operating distance from said nozzle means, saidnozzle means providing a gas chamber having a plasma outlet opening,means for flowing gas into said chamber, means for flowing an arccurrent between said electrode and nozzle means, and means forrelatively displacing said electrode and nozzle means between said firstand second positions, whereby a starting arc is created as saidelectrode and nozzle means move from said first toward said secondposition, the improvement comprising: said means for relativelydisplacing said electrode and nozzle means including means selectivelyoperable for relatively displacing said electrode and nozzle means fromsaid second position toward said first position during the flow of gasinto said chamber and independent of the flow of arc current betweensaid electrode and nozzle means.
 2. A torch according to claim 1,wherein said means for relatively displacing said electrode and nozzlemeans includes piston means displaceable in opposite directions relativeto said nozzle means, said electrode being connected to said pistonmeans for displacement therewith in said opposite directions, saidpiston means having opposite sides each facing in a different one ofsaid opposite directions, one of said opposite directions moving saidelectrode from said second position toward said first position, andmeans for selectively applying an operating force against one of saidopposite sides to displace said piston means in said one direction.
 3. Atorch according to claim 2, wherein said means for selectively applyingan operating force includes means for selectively applying an operatingfluid under pressure against said one side.
 4. A torch according toclaim 3, and means biasing said piston means in the other of saidopposite directions.
 5. A torch according to claim 3, wherein said meansfor selectively flowing an operating fluid includes means forselectively flowing operating fluid against the other of said oppositesides of said piston means to bias said piston means in the other ofsaid opposite directions for moving said electrode from said firstposition toward said second position.
 6. A torch according to claim 5,wherein said operating fluid acting against said one side imposes aforce on said one side greater than the force imposed on said other sideby said operating fluid acting thereagainst, thereby overriding saidbias and moving said piston means in said one direction for moving saidelectrode from said second position toward said first position.
 7. Atorch according to claim 3, wherein said means for selectively flowingan operating fluid against said one side of said piston means includes acontrol chamber and valve means having first and second positionsrespectively connecting said control chamber to a source of saidoperating fluid under pressure and venting said chamber to atmosphere.8. A torch according to claim 7, and tension spring means biasing saidpiston means in the other of said opposite directions.
 9. A torchaccording to claim 7, wherein said means for flowing gas to said gaschamber includes first flow line means for connecting said gas chamberto a source of gas under pressure, and second flow line means betweensaid first flow line means and said control chamber, said valve meansbeing in said second flow line means.
 10. A torch according to claim 9,wherein said electrode has a given diameter, and said one side of saidpiston means has a diameter more than twice said given diameter.
 11. Atorch according to claim 10, and spring means biasing said piston meansin the other of said opposite directions.
 12. A torch according to claim3, wherein said electrode is connected to the other of said oppositesides of said piston means, spring means providing a spring forceagainst said one side of said piston means for biasing said electrodetoward said first position, means providing said one side with a firstreaction surface area, said other side having a second reaction surfacearea, said means for flowing gas into said chamber including means forflowing said gas at a pressure against said second reaction surface tocreate an opening force thereagainst greater than said spring force formoving said electrode from said first to said second position, and saidmeans for selectively flowing an operating fluid against one of saidopposite sides including means for flowing said operating fluid at apressure against said first reaction surface to create a closing forcethereagainst, said closing force supplementing said spring force to movesaid electrode from said second to said first position against saidopening force.
 13. A torch according to claim 12, wherein said springforce and closing force together are at least 1.5 times said openingforce.
 14. A torch according to claim 12, wherein said one side of saidpiston means has a given area and said first reaction surface area isless than said given area.
 15. A torch according to claim 12, whereinsaid means providing said one side with a first reaction surface areaincludes collapsible wall means sealingly connected to said one side.16. A torch according to claim 15, wherein said collapsible wall meansincludes bellows means.
 17. A torch according to claim 15, wherein saidcollapsible wall means encloses said spring means.
 18. A torch accordingto claim 2, wherein said means for selectively applying an operatingforce against one of said opposite sides includes spring means.
 19. Atorch according to claim 2, wherein said electrode is connected to theother of said opposite sides of said piston means, said means forflowing gas into said chamber including means for flowing said gas underpressure against said other side of said piston means to create a firstforce for displacing said electrode from said first to said secondposition, and said means for selectively applying an operating forceincluding means for applying a second force overriding said first forcefor moving said electrode from said second to said first position.
 20. Atorch according to claim 19, wherein said means for applying said secondforce includes electrically activated drive member means, and meansincluding control means for activating and deactivating said drivemember means.
 21. A torch according to claim 20, wherein said means foractivating and deactivating said drive member means includes solenoidmeans.
 22. A torch according to claim 21, wherein said control meansincludes means for intermittently energizing and deenergizing saidsolenoid means.
 23. A torch according to claim 2, wherein said means forselectively applying an operating force against one of said oppositesides of said piston means includes spring means, said electrode beingon the other of said opposite sides, whereby said spring means biasessaid electrode from said second position to said first position, anoperating fluid chamber separate from said gas chamber, said pistonmeans being in said operating fluid chamber, valve means having firstand second positions respectively connecting and disconnecting saidfluid chamber with a source of operating fluid under pressure, saidoperating fluid in said first position applying a force against saidother side of said piston means to move said electrode from said firstto said second position against said operating force, and means toselectively shift said valve means between said first and secondpositions.
 24. A torch according to claim 23, wherein said means toshift said valve means includes electrically operable solenoid means.25. A torch according to claim 24, wherein said solenoid means includesa plunger connected to said valve means, said solenoid means having adeenergized mode in which said plunger positions said valve means in oneof said first and second positions thereof and an energized mode inwhich said plunger positions said valve means in the other of said firstand second positions thereof.
 26. A torch according to claim 25, whereinsaid plunger positions said valve means in said first position thereofin said deenergized mode of said solenoid means.
 27. A torch accordingto claim 26, and spring means biasing said valve means to said firstposition thereof.
 28. A torch according to claim 1, wherein said meansfor relatively displacing said electrode and nozzle means compriseselectrically operable solenoid means including plunger meansdisplaceable in opposite directions relative to said nozzle means, saidelectrode being connected to said plunger means for movement therewith,said solenoid means having a deenergized mode in which said plungermeans positions said electrode said spaced operating distance from saidnozzle means and an energized mode in which said plunger positions saidelectrode in contact with said nozzle means, and means for selectivelyshifting said solenoid means between said deenergized mode and saidenergized mode.
 29. A torch according to claim 28, wherein said solenoidmeans includes means for biasing said plunger means to position saidelectrode said spaced operating distance from said nozzle means in saiddeenergized mode.
 30. In a plasma torch comprising relativelydisplaceable electrode and nozzle means having first and secondpositions relative to one another in which said electrode respectivelycontacts said nozzle means and is spaced an operating distance from saidnozzle means, said nozzle means providing a gas chamber having a plasmaoutlet opening, means for flowing gas into said chamber, and means forcreating a starting arc by relatively displacing said electrode andnozzle means from said first toward said second position, theimprovement comprising: a system for controlling said means for creatinga starting arc including means for creating a voltage across saidelectrode and said nozzle means, detecting means for detecting a closedcircuit condition when said electrode and nozzle means are in said firstposition, and enabling means for enabling said means for creating astarting arc only if said detecting means detects a closed circuitcondition when said electrode and nozzle means are in said firstposition.
 31. A torch according to claim 30, wherein said detectingmeans includes means for detecting an open circuit condition when saidelectrode and nozzle means are in said second position, means forsequentially displacing said electrode and nozzle means from one of saidfirst and second positions to the other, and said enabling meansenabling said means for creating a starting arc only if said detectingmeans detects a sequence of open and closed circuit conditionscorresponding to said one and said other positions.
 32. A torchaccording to claim 31, wherein said one position is an initial positionand said means for enabling includes means for disabling said means forcreating a starting arc when said detecting means does not detect theone of said open and closed circuit conditions corresponding to saidinitial position.
 33. A torch according to claim 31, wherein said meansfor sequentially displacing said electrode and nozzle means includesmeans for holding said electrode and nozzle means in each of said firstand second positions for a predetermined period of time.
 34. A torchaccording to claim 31, wherein said means for sequentially displacingsaid electrode and said nozzle means comprises drive means includingfirst drive means for relatively displacing said electrode and nozzlemeans from said first position to said second position and second drivemeans for relatively displacing said electrode and nozzle means fromsaid second position to said first position, and control meansresponsive to said detecting means for controlling operation of saiddrive means.
 35. A torch according to claim 34, wherein said controlmeans is responsive to said detecting means detecting a closed circuitcondition in said first position of said electrode and nozzle means toactuate said drive means for said first drive means to relativelydisplace said electrode and nozzle means to said second position and isresponsive to said detecting means detecting an open circuit conditionin said second position of said electrode and nozzle means to actuatesaid drive means for said second drive means to relatively displace saidelectrode and nozzle means to said first position.
 36. A torch accordingto claim 35, wherein said means for creating a starting arc includespower supply means, said second position of said electrode and nozzlemeans being an initial position, and said control means being responsiveto said detecting means detecting a closed circuit condition in saidfirst position of said electrode and nozzle means to cause an arcingvoltage to be applied between said electrode and nozzle means from saidpower supply means and to actuate said drive means for said first drivemeans to relatively displace said electrode and nozzle means back tosaid second position for creating said starting arc.
 37. A torchaccording to claim 36, wherein said electrode and nozzle means are in anelectrical circuit connected to said power supply means, switch meansfor opening and closing said circuit, and said control means includingmeans for opening said switch means when said detecting means fails todetect either one of an open circuit condition in said initial positionof said electrode and nozzle means and a closed circuit condition insaid first position thereof.
 38. A torch according to claim 37, whereinsaid control means includes means for delaying actuation of said drivemeans for a predetermined period of time in each of said first andsecond positions of said electrode and nozzle means.
 39. A torchaccording to claim 35, wherein said means for creating a starting arcincludes power source means, said first position of said electrode andnozzle means being an initial position, said control means beingresponsive to said detecting means detecting an open circuit conditionin said second position of said electrode and nozzle means to actuatesaid drive means for said second drive means to relatively displace saidelectrode and nozzle means back to said first position and to cause anarcing voltage to be applied between said electrode and nozzle meansfrom said power source and to thereafter actuate said drive means forsaid first drive means to relatively displace said electrode and nozzlemeans back to said second position for creating said starting arc.
 40. Atorch according to claim 39, wherein said electrode and nozzle means arein an electrical circuit connected to said power supply means, switchmeans for opening and closing said circuit, and said control meansincluding means for opening said switch means when said detecting meansfails to detect either one of a closed circuit condition in said initialposition of said electrode and nozzle means and an open circuitcondition in said second position thereof.
 41. A torch according toclaim 40, wherein said control means includes means for delayingactuation of said drive means for a pre-determined period of time ineach of said first and second positions of said electrode and nozzlemeans.
 42. A torch according to claim 35, wherein said control meansincludes means for delaying actuation of said drive means for apre-determined period of time in each of said first and second positionsof said electrode and nozzle means.
 43. A method of testing a plasmatorch prior to starting thereof, said torch comprising relativelydisplaceable electrode and nozzle members having first and secondpositions relative to one another in which said electrode respectivelycontacts said nozzle and is spaced an operating distance from saidnozzle, said method comprising: applying a test voltage between saidelectrode and nozzle, sensing for a closed circuit condition when saidelectrode and nozzle are in said first position, and enabling creating astarting arc between said electrode and nozzle only when a closedcircuit condition is sensed with said electrode and nozzle in said firstposition.
 44. The method according to claim 43, including the furthersteps of sequentially displacing said electrode and nozzle from one ofsaid first and second positions to the other, sensing for an opencircuit condition when said electrode and nozzle are in said secondposition, and enabling creating a starting arc between said electrodeand nozzle only when a sequence of open and closed circuit conditionscorresponding to said one and said other position of said electrode andnozzle is sensed.
 45. The method according to claim 44, and sensing foreach said closed and open circuit condition for a pre-determined periodof time.
 46. The method according to claim 44, wherein said nozzlemember provides a gas chamber having a plasma opening, and said methodfurther includes flowing an arc gas to said gas chamber.
 47. The methodaccording to claim 46, wherein the flow of arc gas is initiated prior tosequentially displacing said electrode and nozzle.
 48. The methodaccording to claim 46, wherein the flow of arc gas is initiatedconcurrently with initiating sequential displacement of said electrodeand nozzle.
 49. The method according to claim 46, wherein the flow ofarc gas is initiated after initiating sequential displacement of saidelectrode and nozzle.
 50. The method according to claim 44, furtherincluding the step of applying a predetermined engaging force betweensaid electrode and nozzle in said first position thereof.
 51. The methodaccording to claim 43, further including the step of applying apredetermined engaging force between said electrode and nozzle in saidfirst position thereof.
 52. The method according to claim 43, andsensing for said closed circuit condition for a pre-determined period oftime.
 53. The method according to claim 43, wherein said first positionis an initial position, and including the additional steps of relativelydisplacing said electrode and nozzle from said initial position to saidsecond position, sensing for an open circuit condition when saidelectrode and nozzle are in said second position, relatively displacingsaid electrode and nozzle from said second position back to said initialposition, and enabling creating a starting arc when an open circuitcondition is sensed in said second position followed by a closed circuitcondition being sensed in said initial position.
 54. The methodaccording to claim 53, and sensing for a closed circuit condition whensaid electrode and nozzle are in said initial position prior to relativedisplacement thereof to said second position.
 55. The method accordingto claim 54, and sensing for each said closed and open circuit conditionfor a pre-determined period of time.
 56. The method according to claim43, wherein said second position is an initial position, and relativelydisplacing said electrode and nozzle from said initial position to saidfirst position, and then sensing for said closed circuit condition insaid first position.
 57. The method according to claim 56, and sensingfor an open circuit condition when said electrode and nozzle are in saidinitial position prior to relative displacement thereof to said firstposition.
 58. The method according to claim 43, wherein said secondposition is an initial position, and including the further steps ofsequentially relatively displacing said electrode and nozzle from saidinitial position to said first position and back to said initialposition at least twice, sensing for an open circuit condition each timesaid electrode and nozzle are in said initial position, sensing for aclosed circuit condition each time said electrode and nozzle are in saidfirst position, and enabling creating of a starting arc when acontinuous sequence of open and closed circuits is sensed during thesequential relative displacement of said electrode and nozzle.
 59. Themethod according to claim 43, wherein one of said first and secondpositions of said electrode and nozzle is an initial position, andincluding the further steps of relatively displacing said electrode andnozzle between said initial position and the other of said first andsecond positions a pre-determined number of times to a final position inwhich said electrode and nozzle are in said first position, sensing fora closed circuit condition each time said electrode and nozzle are insaid first position, and enabling creating of a starting arc when aclosed circuit condition is sensed in said final position of saidelectrode and nozzle.
 60. The method according to claim 59, and sensingfor an open circuit condition each time said electrode and nozzle are insaid second position, and enabling creating of a starting arc only whensaid sensed closed circuit condition in said final position is precludedby a sensed open circuit condition in the second position of saidelectrode and nozzle immediately preceding said final position.
 61. Themethod according to claim 59, and sensing for an open circuit conditioneach time said electrode and nozzle are in said second position, andenabling creating of a starting arc only when said sensed closed circuitcondition in said final position is preceded by a continuous sequence ofopen and closed circuit conditions sensed during the relativedisplacement of said electrode and nozzle between said initial and otherpositions.
 62. In a plasma torch comprising relatively displaceableelectrode and nozzle means having first and second positions relative toone another in which said electrode respectively contacts said nozzlemeans and is spaced an operating distance from said nozzle means, saidnozzle means providing a gas chamber having a plasma outlet opening,means for flowing arc gas into said chamber, means for flowing an arccurrent between said electrode and nozzle means, and means forrelatively displacing said electrode and nozzle means between said firstand second positions, whereby a starting arc is created as saidelectrode and nozzle means move from said first toward said secondposition, the improvement comprising: means for creating a first forcefor holding said electrode and nozzle means in said second position, andmeans for selectively creating a second force for overriding said firstforce and relatively displacing said electrode and nozzle means fromsaid second position to said first position against said first force.63. A torch according to claim 62, wherein said means for relativelydisplacing said electrode and nozzle means includes piston meansdisplaceable in opposite directions relative to said nozzle means andhaving opposite sides, said electrode being connected to one of saidopposite sides of said piston means for displacement therewith in saidopposite directions, and said means for selectively creating said secondforce including means for imposing said second force against the otherof said opposite sides of said piston means.
 64. A torch according toclaim 63, wherein said means for selectively creating said second forceincludes means for selectively applying fluid under pressure against theother of said opposite sides of said piston means.
 65. A torch accordingto claim 63, wherein said means for creating said first force includesspring means.
 66. A torch according to claim 63, wherein said means forcreating said first force includes means for applying fluid underpressure against said one side of said piston means.
 67. A torchaccording to claim 63, wherein said means for selectively creating saidsecond force includes chamber means for said piston means, said otherside of said piston means being in said chamber means, and valve meanshaving first and second positions respectively connecting said chambermeans to a source of fluid under pressure and venting said chamber meansto atmosphere.
 68. A torch according to claim 67, wherein said means forcreating said first force includes tension spring means connected tosaid other side of said piston means.
 69. A torch according to claim 67,wherein said means for flowing arc gas to said gas chamber includesfirst flow line means for connecting said gas chamber to a source of arcgas under pressure, and second flow line means between said first flowline means and said chamber means for said piston means, said valvemeans being in said second flow line means.
 70. A torch according toclaim 69, wherein said electrode has a given diameter, and said otherside of said piston means has a diameter more than twice said givendiameter.
 71. A torch according to claim 63, wherein said means forselectively creating said second force includes means for selectivelyapplying fluid under pressure against the other of said opposite sidesof said piston means, and spring means providing a spring force againstsaid other side of said piston means.
 72. A torch according to claim 71,wherein said means for creating said first force includes means forflowing arc gas into said gas chamber against said one side of saidpiston means, said first force being greater than said spring force. 73.A torch according to claim 72, wherein said second force is at least 1.5times said first force.
 74. A torch according to claim 72, wherein saidother side of said piston means has a given area, and means providingsaid other side with a reaction surface having an area less than saidgiven area.
 75. A torch according to claim 74, wherein said meansproviding said other side with a reaction surface includes collapsiblewall means sealingly connected to said other side.
 76. A torch accordingto claim 75, wherein said collapsible wall means includes bellows means.77. A torch according to claim 75, wherein said collapsible wall meansencloses said spring means.
 78. A torch according to claim 63, whereinsaid means for flowing arc gas into said gas chamber includes means forflowing said arc gas under pressure against said one side of said pistonmeans to create said first force.
 79. A torch according to claim 78,wherein said means for selectively creating said second force includeselectrically activated drive member means, and means including controlmeans for activating and deactivating said drive member means.
 80. Atorch according to claim 79, wherein said means for activating anddeactivating said drive member means includes electrical coil means. 81.A torch according to claim 80, wherein said control means includes meansfor intermittently energizing and deenergizing said coil means.
 82. Atorch according to claim 62, wherein said means for relativelydisplacing said electrode and nozzle means comprises electricallyoperable solenoid means including plunger means displaceable in oppositedirections relative to said nozzle means, said electrode being connectedto said plunger means for movement therewith, said solenoid means havinga deenergized mode in which said plunger means positions said electrodein said second position and an energized mode in which said plungermeans positions said electrode in said first position, and said solenoidmeans including said means for creating said first force and said meansfor creating said second force.
 83. A torch according to claim 82,wherein said means for creating said first force includes means forbiasing said plunger means to position said electrode in said secondposition in said deenergized mode of said solenoid means.
 84. A torchaccording to claim 83, wherein said solenoid means includes electriccoil means for displacing said plunger means in said energized mode ofsaid solenoid means, said coil means and plunger means providing saidmeans for creating said second force.